Materials that emit light when subjected to electron bombardment have long been used to form screens in cathode ray tubes and, more recently, in Field Emission Displays (FEDs). Since these materials, known as phosphors, are relatively poor electrical conductors, it is necessary to back them with a conductive layer to prevent the accumulation of any charge. This layer then serves as the anode of the display system.
Two types of phosphor screen anode are in use. In the first type a layer of a transparent conductor (typically indium tin oxide or ITO) is first deposited onto a transparent substrate and the phosphor layer is then formed on top of the ITO. This design has the advantage of not interfering with the electrons on their way to the phosphor but its efficiency is limited by the fact that any light emitted by the phosphor, in a direction away from the substrate, is lost to the display.
The second type of anode is designed to overcome this deficiency. Instead of ITO, the phosphor layer is formed directly on the substrate following which it is covered with a thin layer of metal, typically aluminum. This metallic anode is thin enough that the electrons are able to pass right through it. Once they reach the phosphor the electrons emit light as in the first type but now light emitted in a direction away from the substrate is reflected by the aluminum layer and is no longer lost to the display.
In FIG. 1 we illustrate the method that has been favored in the prior art for the manufacture of displays having aluminum anodes. Phosphor layer 2 is first laid down on the top surface of substrate 1. Since layer 2 is made up of a large number of individual phosphor particles, its top surface is rough and any metallic film deposited on it will follow the contours of the phosphor layer and therefor also be rough. A rough topography for the underside of the aluminum anode is undesirable because reflection from it will be diffuse making for a less crisp display.
In order obtain an aluminum film with a smooth underside, a common practice was to lay down a layer of lacquer (marked as 3 in FIG. 1) to act as a planarizing medium. Being liquid, the lacquer soon settled into a planar upper surface, following which it was dried and the aluminum film was then deposited directly onto it. Removal of the lacquer layer was then effected by heating in oxygen at around 450.degree. C., leaving behind an aluminum layer having a clean and smooth undersurface.
While the lacquer method decribed above works, a major disadvantage associated with it is that the lacquer is highly toxic so special precautions need to be taken during its use. This slows down the manufacturing process and adds to the cost of the final product.
We have recently filed a patent application (application Ser. No. 08/789,216, now U.S. Pat. No. 5,843,534, on Jan. 24, 1997 by D. A. Chung and J. Y. Lu) which teaches an alternative to the lacquer approach, namely formation of a phosphor slurry which serves the double purpose of facilitating application of the phosphor as well as smoothing out the top surface prior to aluminum deposition. The solvent and binder used to form the slurry are non-toxic and are conveniently removed by firing in oxygen.
The slurry formulation disclosed in application Ser. No. 08/789,216, now U.S. Pat. No. 5,843,534 requires firing at temperatures in excess of 500.degree. C. (about 520.degree. C. being typical). This has the undesired side effect of causing some roughening of the aluminum layer's surfaces which in turn causes some of the light emitted away from the substrate to be reflected in unpredictable directions, reducing the net brightness of the display. The present invention is concerned with preserving the advantages of using a non-toxic slurry but without the aluminum surface roughening problem associated with firing at temperatures in excess of 500.degree. C.
There have been reports in the prior art of attempts to solve similar problems (although none resemble the approach taken by the present invention). For example, Hayashi et al. (U.S. Pat. No. 4,857,429 August 1989) provide optical contact between a powder and a substrate by filling the interstices between the particles with an inorganic material whose refractive index matches that of the particles. Barrow et al. (U.S. Pat. No. 4,751,427 June 1988) describe an electroluminescent device comprising several phosphor layers sandwiched between two layers of aluminum oxide. Power to the device is applied through an aluminum electrode on one face and a transparent conducting electrode on the other face. Trond et al. (U.S. Pat. No. 4,568,479 February 1986) show how a phosphor slurry may be made photosensitive by mixing in suitable additives.